1. Field of the Invention
A conventional telephone set is connected to a double-conductor telephone line, whose two conductors carry a composite signal comprising both a signal transmitted by the telephone and a signal received by the telephone.
2. Description of the Prior Art
In order to be able to hear the voice of the other person in a receiver (received signal), the receiver must be connected to the line. However, if it is directly connected, the composite signal, i.e. not only the received signal but also the transmitted signal will be heard in the receiver. Thus, the user of the telephone hears himself speak in the receiver, which is disturbing. Moreover, there is a risk of microphony between the mouthpiece and the receiver. Thus, a so-called "antilocal" circuit is provided, which is a microphone or mouthpiece signal suppression circuit receiving the transmitted signal (mouthpiece signal) and the composite signals present on the line in order to subtract the first from the second and obtain a signal, which is essentially the received signal, and which can be tranmsitted to the receiver.
Thus, a priori, the suppression circuit essentially comprises a subtracter, which receives the composite signal on the line and the signal from the mouthpiece.
Thus, it is necessary to also provide impedance matching, in order that the subtraction takes place on the signal circulating in the equivalent impedances, otherwise the reflections produced would deteriorate the quality of the subtraction.
Two examples of conventional microphone or mouthpiece signal suppression circuits are shown respectively in FIGS. 1 and 2.
In FIG. 1, the telephone line is represented by two conductors 10 and 12 reaching the two terminals 14 and 16 of the telephone. Terminal 16 is considered to be the earth terminal.
The mouthpiece of the telephone hand set is designated by reference numeral 18 and the receiver by reference numeral 20. The mouthpiece is connected to the input of an amplifier 22, which supplies a mouthpiece signal M. The latter is applied to the telephone line by an amplifier 24, which can have a gain K. The composite signal S present on conductor 16 consists of a combination of mouthpiece signal and a received signal and which is heard through receiver 20.
The impedance of the telephone line is symbolically represented by a terminating impedance 26, connected to the end of the line and whose value is Z.
The mouthpiece signal suppression circuit takes account of this impedance and, in FIG. 1, comprises a compensation impedance 28 connected between the terminal 14 receiving the telephone line and a positive input of a subtractor 30, whereof a negative input also receives the mouthpiece signal M and whose output drives an amplifier 32 connected to the receiver 20.
The compensation impedance has a structure (resistor and capacitor network) taking account of the impedance value Z of the line, as well as the gain K of the amplifier 24 towards the line.
FIG. 2 shows a constructional variant, in which the same elements are designated by the same references. The only difference is that the positive input of subtractor 30 is connected directly and not via a compensation impedance to conductor 10 of the telephone line (terminal 14). The compensation impedance 28 (which does not have the same structure as in FIG. 1) is replaced at the output of the mouthpiece amplifier 22, between said output and the second terminal 16 (earth) of the telephone line.
It is also possible to provide other connections (e.g. bridge connections). All these connections suffer from a major disadvantage, namely the satisfactory operation of the suppression circuit requires a precise matching between the compensation impedance value and the line impedance value. However, the line impedance varies greatly with the line length and type (diameter, etc). It is therefore necessary to adjust or modify the compensation impedance in order to individually match it to each telephone, as a function of its geographical position with respect to a telephone exchange. This obviously leads to additional manufacturing costs (regulatable elements) and higher installation costs (controls to be carried out during the installation). It is therefore preferred to use an impedance value corresponding to a line of average length, but the results are far from satisfactory.
It has also been proposed to switch the compensation impedance between two values, one corresponding to shorter lines and the other to longer lines, switching taking place automatically on the basis of a direct current and/or a d.c. voltage measurement on the telephone line, because said current or voltage can be a good indication of the line impedance.
However, this switching between two impedance values still gives unsatisfactory results. In order to improve these results, it is necessary to have a much more precise switching between the numerous possible impedance values, which would make the circuitry of the telephone set much more complicated. It would also be conceivable to construct a continuously variable impedance (variable resistor, variable capacitance diode) as a function of an electrical signal, but this would lead to design and construction problems.